The present disclosure relates generally to an analog-to-digital converter (ADC) used in electronic circuits, and more particularly to a method and an apparatus for improving the performance of ADC's deploying a sigma-delta modulator (SDM).
The SDM is a well known architecture for over-sampled, analog-to-digital data conversion due to its advantages in silicon area, lower power, and the noise-shaping abilities. The SDM may be implemented in either discrete form using switched capacitor circuits or in continuous time, using switched resistors or current sources. It is well known that over-sampling improves SDM performance, e.g., by improving signal-to-noise ratio (SNR). Double sampling is a well known technique that increases the maximum sample rate of a sampled data system without utilizing faster components. The increase in the effective sampling rate is achieved by interleaving two sampling states in the time domain. In one such implementation, the data may be sampled on the rising edge and the falling edge of the same clock, using two different quantizers which feed the output result in a feedback loop to two different digital-to-analog converters (DAC's). The effective sampling rate of a double-sampled SDM is doubled while the sub-components still operate at the more relaxed speed requirements.
A key limitation in a double-sampled SDM, however, arises from a mismatch of feedback elements, e.g., DAC's, included in the feedback path. Any mismatch in the feedback path modulates the digital output data stream generated by the quantizer(s). Since the quantizer output in a sigma-delta modulator has, in addition to the input signal, significant high frequency components, the modulation of mismatch with this high frequency signal causes an in-band error component, which directly manifests itself at the output of the ADC. This phenomenon is often detrimental to the SDM performance. For example, in one simulated application for a traditional SDM, an intentional 1% mismatch in the feedback DAC increased the in-band noise floor considerably, and resulted in a SNR loss of about 10 dB.
A traditional solution for reducing the in-band noise floor utilizes three redundant feedback DAC's that are dynamically chosen in a manner that shapes the mismatch energy out of the band of interest. However, many traditional solutions often rely on the integrity of sampled data collected over a plurality of rising and falling edges of the clock. Non-idealities of electronic components used in a traditional SDM often leads to degradation in the in-band noise floor and may even cause instability in case of low open-loop gain of an operational amplifier. Therefore, a need exists to provide an improved SDM that is capable of handling non-idealities and mismatches in the electronic components used in the SDM. In addition, the improved SDM should provide an improved performance, e.g., as measured in terms of the SNR and the in-band noise floor in the band of interest, compared to the traditional SDM.